The present invention relates to processes for reducing the amount of nitrogen oxides in flue gases emitted into the atmosphere from combustion systems and, in particular, to a method for treating nitrogen oxides in combustion gas waste streams using a new method for determining and controlling the precise amount of ammonia necessary to substantially reduce and/or eliminate nitrogen oxide emissions using selective catalytic reduction (“SCR”).
Nitrogen oxide compounds form as by-products of the imperfect high-temperature combustion are considered major pollutants emitted by combustion sources. The exhaust gases invariably include nitric oxide (NO) and nitrogen dioxide (NO2) with the total NO+NO2 concentration nominally referred to as “NOx.” In recent years, nitrogen oxides have become the subject of increasing public concern due to their potential toxicity. NOx compositions are also known to be chemical precursors of acid rain or photochemical smog and contribute to the “greenhouse” effect. NOx also plays a role in forming ground-level ozone associated with asthma and other respiratory ailments.
Thus, NOx emissions have become the subject of increasingly stringent federal and state regulations limiting the amount permitted in effluent gas vented to the atmosphere. Current in-force pollution control regulations also provide the incentive for industries to find improved, lower cost processes for substantially reducing or eliminating NOx emissions.
In an ideal combustion gas treatment system, NOx compounds are distributed uniformly in the exhaust steam and treated with a catalyst to create unregulated compounds (e.g., nitrogen) that can then be released into the atmosphere. In theory, NOx treatment processes should form stoichiometrically zero NOx gases leaving the catalyst bed. Unfortunately, various practical limitations prevent the achievement of either uniform NOx concentrations or zero NOx emissions in treated exhaust gases. One reason for the lower NOx conversion efficiencies is the spontaneous partial reaction of ammonia with other compounds present in the exhaust streams when the ammonia is first introduced. The presence of such compounds can result in the inefficient use of ammonia and/or the ineffective reduction of NOx in the system. In addition, shifts in exhaust stream compositions can cause stoichiometrically incorrect amounts of ammonia to be present, thereby increasing the cost of reducing NOx to “safe” compounds and creating the possibility that excess ammonia (typically referred to as “ammonia slip”) may be released into the atmosphere.
A known process for treating NOx in exhaust streams uses selective catalytic reduction (“SCR”) to reduce NOx to nitrogen gas using ammonia as the reducing agent. However, because of ammonia's hazardous nature, its use in an SCR system presents additional environmental problems that must be addressed. As federal and state regulatory agencies continue to drive NOx emission limits down, other regulations have also reduced the permissible levels of NH3 that can be emitted into the atmosphere. The presence of unused ammonia in SCR processes also raises concerns over the overall cost for treating NOx emissions.
One difficulty in controlling NH3 emissions using SCR relates to the design of heat recovery steam generator (“HRSG”) systems used in combined cycle power generation plants. Most HRSG systems have the ability to adapt to changing gas flow rates and/or a non-uniform distribution of exhaust gas components, including NOx. However, HRSG exhaust compositions and temperatures will vary widely depending on the upstream load. Although some SCR processes allow for “tuning” of the ammonia concentration in the HRSG exhaust gas by monitoring the NOx concentration downstream of the SCR, previous attempts to treat non-uniform concentrations of NOx in an HRSG flow pattern (“spatial distribution”) by adding ammonia before the gas contacts the SCR catalyst have met with very limited success. Thus, difficulties remain in creating and maintaining an adequate spatial distribution of ammonia in exhaust gas flows treated on a continuous basis.
Most combined cycle SCR units operate downstream of a high pressure heat exchanger that reduces the HRSG exhaust gas temperature down to a level of between 600° F. and 750° F. The lower exhaust temperatures range is selected to ensure a high percentage NOx reduction, low ammonia emissions (ammonia slip), and protect the SCR catalyst from degradation due to exposure over time to high temperatures, e.g., in the 825° F. to 850° F. range, which can cause irreversible damage to the SCR catalyst. Most catalysts useful in reducing NOx must also operate in the 600° F. and 750° F. range to avoid any oxidation of the ammonia to form additional NOx. Any such reactions forming additional NOx necessarily increase the amount of ammonia required by the SCR process and reduce the NOx removal efficiency of the entire system. Lower exhaust gas temperatures also avoid oxidizing sulfur dioxide in the HRSG exhaust to form SO3, which in turn can result in an ammonia sulfate accumulation within the heat recovery steam generator.
Thus, despite some improvements in treating HRSG exhaust gases, a need still exists to maintain a more uniform distribution of ammonia at the SCR inlet in order to reduce the possibility of parasitic reactions or the presence of excess unreacted ammonia following SCR treatment. For a given catalyst and reactor design, the amount of unreacted ammonia being used depends in major part on the exhaust gas temperature, catalyst, exhaust gas flow distribution and the local NH3 to NOx ratio. For most ammonia-SCR catalysts, optimum high NOx reduction and low ammonia slip are achieved over a relatively narrow temperature range. Preferably, an SCR control system should reduce the ammonia slip and complete the NOx destruction by adjusting the NH3 to NOx ratio based on the exhaust temperature and detected NO to NO2 ratio. If the NO to NO2 ratio falls below some value (typically 1.0), the SCR may not be adequate in size to achieve the required reduction in NOx emissions.
Another process factor effecting NOx removal, particularly in treating gas turbine exhausts, relates to age and reactivity of the SCR catalyst. SCR catalysts increase the reaction rate for reducing NOx to nitrogen in the feed to the SCR without the catalyst being consumed in the reaction. As a result, the equilibrium products for a steady state reaction do not change significantly. NH3 and NOx diffuse into the catalyst pores and are adsorbed onto active catalyst sites. Nevertheless, catalyst “poisons” or material blocking the pores or site can effectively deactivate catalyst sites over time.
One last environmental concern of current ammonia-based SCR systems involves the use of auxiliary duct burners. The exhaust gases from gas turbine engines contain a significant quantity of heat energy that can be used to generate steam (and then electricity) using a steam turbine. However, if the heat requirement of the system exceeds that available from the gas turbine exhaust alone, many plants now use supplementary firing in the form of a downstream duct burner positioned between the gas turbine and a waste heat boiler. Most duct burner designs mix the turbine exhaust gases with additional fuel in an effort to improve flame stability and ensure a clean combustion with low NOx emissions. Nevertheless, the presence (or absence) of a duct burner can have significant impact on the ultimate performance of a downstream SCR unit designed to remove all NOx from the system.
Thus, numerous problems and challenges remain for NOx emission control of gas turbine engines, including the need to develop a method that maximizes the reduction of NOx using ammonia as the primary reducing agent in SCR systems while avoiding the use of excess ammonia or the formation of parasitic ammonia reactions that reduce the efficiency of the SCR process by controlling the desired spatial distribution of ammonia to NOx molar ratio at any operating condition.